One proposed structure of the internal combustion engine system makes tentative detection of occurrence of a misfire when a rotational fluctuation Δt of a crankshaft is greater than a predetermined reference value for detecting the misfire (see Patent Document 1). When the tentative detection is made in this system, the system makes a final detection on actual occurrence or non-occurrence of the misfire in the cylinder where the tentative detection of occurrence of the misfire is made. The final detection is made according to whether a rotational fluctuation ΔTN is greater than the product of a rotational fluctuation ΔTN(0) in non-occurrence of a misfire and a coefficient k. The rotational fluctuation ΔTN is a rotational fluctuation of the crankshaft in a power stroke of the cylinder ignited next to the cylinder where the tentative detection of occurrence of the misfire is made.    Patent Document 1: Japanese Patent Laid-Open No. 2005-307945